PHOENIX-7936 EXPLAIN the user's projection on server-driven mutation inner plans

[apurtell]

Server-side UPSERT SELECT and DELETE compile their inner scan as an aggregating SELECT COUNT(1) whose count reports how many rows were touched. Under EXPLAIN (VERBOSE) that internal rewrite leaked as a misleading PROJECT COUNT(1) line (and serverProject: ["COUNT(1)"] in JSON) on the inner scan rather than describing the mutation. ExplainTable factors the per-scan PROJECT naming into a reusable static projectedColumnNames(RowProjector) and adds overrideMutationProject, which rewrites the serverProject attribute with the user-facing projection for UPSERT SELECT and DELETE.

Co-authored-by: Claude Opus 4.8[1m] noreply@anthropic.com

[Copilot]

Pull request overview

This PR improves EXPLAIN (VERBOSE) output for server-side UPSERT SELECT and DELETE by preventing the internal SELECT COUNT(1) rewrite (used only to compute mutation row counts) from leaking into the inner scan’s reported PROJECT/serverProject, and instead surfacing a user-facing projection.

Changes:

• Factored VERBOSE PROJECT rendering into reusable ExplainTable.projectedColumnNames(RowProjector).
• Added ExplainTable.overrideMutationProject(...) to rewrite the inner plan’s PROJECT line and serverProject attributes for mutation plans.
• Wired the rewrite into server-side mutation explain plans in UpsertCompiler and DeleteCompiler.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
phoenix-core-client/src/main/java/org/apache/phoenix/iterate/ExplainTable.java	Factors projection-name rendering and adds a helper to override mutation PROJECT/serverProject to be user-facing.
phoenix-core-client/src/main/java/org/apache/phoenix/compile/UpsertCompiler.java	Applies the mutation projection override so VERBOSE explain reflects the user’s SELECT projection for server-side UPSERT SELECT.
phoenix-core-client/src/main/java/org/apache/phoenix/compile/DeleteCompiler.java	Applies the mutation projection override so VERBOSE explain reflects the delete’s row-identity projection rather than the internal count.

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